1. Field of the Invention
The present invention relates to a method for the production of high-purity ionic liquids. In particular, the instant invention is directed to making high-purity ionic liquids to prevent the destruction of the catalyst and the ionic liquid during catalytic reactions. The present invention also relates to a method for the purification of recycled or contaminated ionic liquid media.
2. Description of the Related Art
Ionic liquids are currently investigated for a variety of different applications, e.g., solvent media for homogeneous and heterogeneous catalysis, extraction/separation processes, membrane technology, polymerization media, lubricants, etc. Many of these applications require high-purity ionic liquids. Although the preparation of ionic liquids has been established, certain methods have led to the production of contaminated products. The use of “impure” ionic liquids results in the destruction of the catalyst and leads to the formation of undesired products.
Most of the known ionic liquid preparations utilize acids to introduce the corresponding anions. Although most of the ionic liquids are produced by this procedure, they are often contaminated with trace amounts of acid/base and/or halides. An investigation of commercially available materials showed such contamination. Furthermore, the olefinic starting material can be oligomerized into undesired side-products due to the acid contamination. Other studies have shown that the presence of residual chlorine traces in ionic liquids act as a very potent catalyst poison. Therefore it is highly desirable to have a synthesis method that enables the preparation of high-purity ionic liquids.
In most of the known ionic liquid preparations, an alkylimidazolium or pyridinium halide is treated with an acid to introduce the corresponding anion. In other words, in conventional preparations FIG. 1, equation (a) the imidazolium or pyridinium halide (M+X−) is converted via acid treatment (H+A−) to form the corresponding ionic liquid (M+A−) and an acid (H+X−). FIG. 1, equation (b) shows a typical preparation of the ionic liquid 1-butyl-3-methyl-imidazolium hexafluorophosphate. The imidazolium chloride is dissolved in an aqueous solution and then treated with an equivalent of HPF6 in water. After several hours of stirring, the ionic liquid separates out as a second liquid phase. The resulting phases are separated and the ionic liquid isolated. Further purification of the ionic liquid involved additional washings with water or basic solutions. It is known to be very difficult to remove all the acid traces by either water washings or by neutralization. In both cases, one obtains an ionic liquid that has either acid or base contaminants. In the different methods of using acids for the preparation of ionic liquids, all led to the decomposition of both the catalyst and ionic liquid when applied in catalysis.
With respect to further purification sequences, most of the published methods do not incorporate any further steps. This common practice is also reflected in the low purity of the commercially available materials. Analysis of such samples showed the presence of acid and halide contaminants. Therefore, there is a need in the art for a method for preparing high-purity ionic liquids.
For example, most of the ionic liquids used in hydroformylation catalysis contain anions like tetrafluoroborate or hexafluorophosphate. The usage of slightly acidic ionic liquids—i.e., those containing trace amounts of unreacted starting material, like the acid HPF6—in hydroformylation catalysis initiates the decomposition of the ionic liquid anions and results in the formation of hydrofluoric acid (HF). This very aggressive acid destroys both the catalyst and the ionic liquid. Moreover, the olefinic substrate is oligomerized to undesired side-products.
Hence there is a need in the art for a method to make a high-purity ionic liquid that will not decompose or adversely affect the performance of a reaction catalyst. There is also a need in the art for a method to purify contaminated ionic liquids.